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Transactions of the Society. 
constancy and accuracy of movement could only be attained by such a 
simple positive band-fed apparatus as that to which we have alluded. 
The form we designed to meet the condition is shown in fig. 26. 
The carbons are held in V-clamps sliding upon two rigid vertical 
rods, and their relative positions can be varied by the right and left 
handed screw A; the position of the pair, that is of the arc as a 
whole, being controlled by the screw B. The lamp is mounted upon 
a boss sliding upon a tube, and has movements through the angle of 
inclination and in a line at right angles to the line of collimation. 
Fig. 26. Fig. 27. 
With such mechanism the arc was fed to maintain its truly central 
position and constant condition, in reference to the graduations upon 
the screen, and any difficulty in its use entirely disappeared, and the 
reproduction, even with the highest powers, of the best photographic 
results became a matter of ease. 
It remained to design means whereby the indications of the pro- 
jected image could be observed in a smaller compass, and we were 
ultimately led to adopt a pin-hole casting an image upon a screen of 
ground glass provided with the necessary reference lines. Such an 
